1. Field of the Invention
The present invention relates generally to optical fibers, and particularly to a method for identifying optical fibers which exhibit elevated levels of polarization mode dispersion (PMD).
2. Technical Background
PMD is an important factor in the design of state-of-art fiber optic transmission systems. The effect of PMD in fiber systems is evident when, after propagating a sufficient distance in the network, one digital pulse may spread in the time domain and become indistinguishable from a nearby pulse. The pulse spreading from PMD can introduce errors into the data transmission, effectively limiting the transmission rate of the pulses or the maximum distance of the concatenated fiber medium.
PMD originates from fiber geometric deformation and stress asymmetry. Without external perturbations, the PMD grows linearly at the rate which corresponds to the level of intrinsic fiber birefringence as the fiber length increases. However, fibers are subject to random external perturbations that induce mode coupling between different polarization modes. The external mode coupling is typically characterized by the frequency of its occurrence 1/h, where h is called mode-coupling length. It has been found that, for fibers in the long length region (fiber having a length l>>h), statistically the fiber PMD is closely related to both the fiber beatlength and mode coupling length,                     PMD        =                              λ                          cL              b                                ⁢                      h                                              (        1        )            where λ is the wavelength of the light, c is the speed of the light, Lb is the beatlength of the fiber as explained in “Polarization Mode Dispersion of Short and Long Single-Mode Fibers”, Journal of Lightwave Technology 9, 821 (1991). Beatlength reflects the intrinsic birefringence that is built into the fiber during the manufacturing process. Mode-coupling length reflects the impact of fiber deployments, and may change as fiber deployment conditions change. The understanding of Eq.(1) has important implications on how PMD is measured and interpreted. Measurement schemes for both fiber beatlength and PMD have been developed.
An assumption behind Eq.(1) is that the fibers are linear birefringent or unspun. In recent years, fiber spinning has been introduced during the fiber draw process to reduce fiber PMD. A significant portion of fibers sold in the market today are spun fibers. For spun fibers, in addition to its dependence on fiber birefringence and random mode coupling, fiber PMD also depends on fiber spinning parameters. For most of the cases when fiber spinning is not operated under optimal condition (e.g. when maximum PMD reduction is achieved), fiber PMD still depends on fiber birefringence or fiber beatlength (before the fiber is spun) and mode-coupling length. However, as explained in “Scaling properties of polarization mode dispersion of spun fibers in the presence of random mode coupling”, OPTICS LETTERS, Vol. 27, No. 18, 1595, (2002), fiber spinning introduces an additional factor |J0(2α/η)| for a sinusoidally spun fiber, where J0 is the zero-order Bessel function, α is the spin magnitude and η is the angular frequency of the sinusoidal profile, that is used to correct Eq.(1).
Fiber manufacturers are interested in providing fibers with uniformly low PMD, particularly for products targeted for high data rate, long-haul transmission systems. Unfortunately, screening fiber PMD on an entire length of fiber directly is a difficult and expensive processing step.
Traditional PMD measurements typically involve obtaining the total differential group delay (DGD) value for the whole fiber under test. When fiber DGD values are elevated beyond a reasonable level, it suggests that at least a portion of the fiber under test carries elevated PMD values, and the fiber is subsequently rejected. Conversely, when the DGD value of a fiber is low, it is natural to assume that fiber PMD is acceptable. However, in reality, the fiber PMD has a distributed nature, and consequently fiber PMD can vary from one segment along the length of the fiber to another. The whole fiber can then be considered as a concatenation of many segments of unperturbed fibers with mode-coupling happening at the junction of fiber segments. Although for a large ensemble of fibers, the overall DGD follows statistical behaviors such as those shown in Eq.(1), for an individual fiber, because of mode coupling, DGD values can be partially canceled from one segment of fiber to another segment of fiber, and the overall fiber could demonstrate a low DGD value. Thus a low value in the total DGD of the fiber under test does not necessarily imply that the fiber has uniformly low PMD values. When such fiber is deployed in the field and the mode-coupling conditions change, there is a high chance that the fiber will exhibit higher DGD values.
PMD measurement in fiber manufacturing facilities often involves measuring a small percentage of fibers, with the frequency of sampling based on process capability. During typical PMD test measurements, fiber samples having a length of about 1 km are wound on a large diameter measurement spool with low tension. This configuration ensures that the induced birefringence and PMD due to bending and winding tension is minimal. Although this type of measurement yields accurate results for the particular segment of fiber under test, it is difficult using this method of screening to deterministically filter out all fibers having unacceptable level of PMD values, due to the distributed nature of PMD. In addition, this form of screening is expensive, as the sampled fibers can not be reused. Therefore, there is a need for more robust screening method that can take the distributed nature of fiber PMD into account.
Accordingly, alternative methods that can conduct the measurements distributedly and non-destructively for identifying fibers with high PMD would be of great value to the industry in that such methods would reduce measurement (quality control) costs, and therefore overall manufacturing costs for low PMD optical fibers.